The present invention relates to a head drive unit for ink-jet recorder and the like, and a method of driving the same.
In ink-jet recording, thermal method and piezoelectric method are the two methods now in use widely. Between these two, the piezoelectric method has a feature that is capable of controlling precisely amount of ink mist and an ejecting spot since it uses a piezoelectric element as an actuator to eject ink mist.
Referring now to the accompanying figures, driving waveforms for an ink-jet head of the piezoelectric method will be described hereinafter.
FIGS. 5A through 5C show head driving waveforms and injecting operation of ink mist. FIG. 5A is a diagrammatic illustration depicting an example of head driving wave as a voltage waveform, FIG. 5B is another diagrammatic illustration depicting the example of head driving wave as a current waveform, and FIG. 5C a diagrammatic illustration depicting changes of an actuator and a meniscus of a head, and appearance of ejected ink mist. Points of time at which driving waveform changes are represented as t1, t2, t3, t4, t5, t6, t7 and t8, voltage values that cause deformation of the actuator as Va, Vb, Vc and Vd, and current values that cause the deformation of the actuator as Ia, Ib, Ic and Id.
Because the actuator consisting of a piezoelectric element is a capacitive load, the waveform shown in FIG. 5B has such a relation to FIG. 5A in that the waveform of FIG. 5A is differentiated.
With reference to the current waveform, described hereinafter pertains to an example of how an ink-jet head is driven.
At the time 0, actuator 82 and meniscus 83 provided in one part 81 of the head are in a flat steady state 91. When the actuator 82 is charged with electric current 1b at the time t1, the actuator 82 begins to deform gradually in a direction of pushing out the meniscus 83. At the time t2, it deforms up to a state marked 92. After this state is maintained until the time t3, the actuator 82 is discharged by electric current Ic until the time t4, to cause the actuator 82 to pull the meniscus 83 back to a state marked 93. After this state is maintained until the time t5, the actuator 82 is charged rapidly by a larger electric current Id than the current Ib until the time t6, so as to cause the actuator 82 to push the meniscus 83 abruptly out to a state marked 94, and to make it eject ink mist 84. This state is held until the time t7 thereafter, and the meniscus 83 is gradually pulled back, and returned to the flat steady state 91 by discharging the actuator 82 by a smaller electric current la than the current Ic until the time t8.
One printing cycle (T) consisting of the above operations is repeated for a number of ties necessary to produce an image.
Described next pertains to the conventional head driving waveform generator circuit which performs the above operations.
FIG. 3 is an example of driving current waveform for the head actuator, as is shown in FIG. 5B. In this example, reference characters t1 through t8 represent the time at which the electric current changes, and numerical values within parentheses under them are time data representing the time (shown in hex number; all data will be shown hereinafter using the hexadecimal number system). Reference characters Ia, Ib, Ic, and Id represent values of the electric current, and numerical values in parentheses next to them are electric current data. Here, a direction in which the electric current flows toward the head actuator is given as positive, another direction where the current flows out of the head actuator as negative, and electric current data when its value is 0 is assigned to be 7F.
In FIG. 3, assuming that the printing cycle (T) is 25.6 microseconds, and time resolution is 0.1 microsecond, the driving current waveform for one printing cycle, when expressed in xe2x80x9celectric current data/time dataxe2x80x9d is shown as follows. That is, 7F/00, 7F/01, 7F/02, . . . , 7F/20, A3/21, A3/22, . . . , A3/49, 7F/4A, . . . , 7F/57, 19/58, 19/60, 7F/61, . . . , 7F/6B, F4/6C, . . . , F4/70, 7F/71, . . . , 7F/86, 42/87, . . . , 42/9E, 7F/9F, . . . and 7F/FF. It becomes data of 256 Bytes.
Two examples of generating the above-described head driving current waveform will be described next. FIG. 6 is an example of block diagram of a head drive unit constructed with a memory.
In FIG. 6, a CPU (not shown in the figure), which controls a system of the ink-jet recorder, writes electric current data 16 in memory 121 using a time as an address, prior to initiating the head drive operation. Counter 1 repeats clearing operation and counting operation for every printing cycle according to the printing operation of the ink-jet recorder. Count data 11 is supplied to the memory 121 as an address, and the electric current data 16 is output from the memory 121. This electric current data 16 is converted into an analog value by DAC 7. An output of the DAC 7 is amplified by the amplifier circuit 8, supplied to head 9, and deforms the actuator 82. Deformation of the actuator causes ejection of ink mist.
Referring now to a block diagram of FIG. 7, another example of head drive unit constructed with a shift register is described.
In FIG. 7, a CPU (not shown in the figure), which controls a system of the ink-jet recorder, writes electric current data 16 for one printing cycle in time-sequential order into shift register 141, prior to initiating the head drive operation. The shift register 141 outputs the electric current data 16 in synchronization with clock according to the printing operation of the ink-jet recorder. A number of registers contained in the shift register 141 is equal to a number of the data for one printing cycle. In addition, since the output is fed back to input, it repeats outputting the head driving waveform in synchronism with the printing cycle.
Using FIG. 8 through FIG. 15, a process of generating the 256 Bytes of head driving current data is described next.
The ink-jet head receives a great influence of an ambient temperature, rise and fall in temperature of ink in particular, upon its performance of ejecting ink mist, i.e., ejecting amount and ejection velocity. It is therefore necessary to make correction of the head driving waveform according to the temperature, in order to maintain the ejecting performance for stable ink mist. The correction can be made in one case by varying only value of the electric current while keeping its timing unchanged, or in another case, by varying both timing and value of the electric current.
Referring to FIG. 8 through FIG. 10, described first pertains to the case of making correction by varying only the current value while not changing the timing. FIG. 8 shows an example of head driving waveforms (waveform 161 in solid line and waveform 162 in dotted line) at different temperatures. FIG. 9 shows an example of driving current value to temperature characteristic necessary to keep constant the ejecting performance of ink mist. Ink requires greater driving energy at lower temperature since its viscosity generally increases. Therefore, value of the electric current increases at low temperature, and decreases at high temperature. This temperature characteristic is non-linear relative to temperature. In a correction data table, 10 points or so of reference data are maintained in general as shown with dark dots in the figure in order to reduce an amount of the data. A value of electric current corresponding to any actual operating temperature is obtained by linear interpolation according to the reference data at both sides adjacent to that temperature.
A flow chart for this process is shown in FIG. 10. Upon start of making a data table for the head driving current, an environmental temperature (operating ambient temperature/ink temperature) is checked (S101), and a data table for reference head driving current is selected (S102). Afterwards, a data table for the head driving current is generated (by linear interpolation relative to the current value) according to the environmental temperature (S103), and generation of the data table for head driving current is completed.
Using FIG. 11 through FIG. 15, described here is the case of making correction by varying both timing and current value. Both the current value and the timing are varied as shown in FIG. 11, depicting an example of head driving waveforms under different temperatures (waveform 221 in solid line and waveform 222 in dotted line). FIG. 12 shows a given operating temperature 243 and reference temperatures 242 and 241 next to the temperature 243. It further shows that a difference in temperature between the operating temperature 243 and the reference temperature 242 is given as TEMP1, and another difference in temperature between the operating temperature 243 and the reference temperature 241 is given as TEMP2.
FIG. 13 is an enlarged illustration showing an encircled portion xe2x80x9cAxe2x80x9d in FIG. 11. With reference to FIG. 13, described now is a case in which a driving waveform for the given operating temperature is obtained from driving waveforms of the reference temperatures. Here, a difference in rise timing between waveform 262 at the reference temperature and waveform 263 at the operating temperature is given as T1, and another difference in rise timing between waveform 261 at the other reference temperature and the waveform 263 at the operating temperature is given as T2. Also, a difference in fall timing between the waveform 262 at the reference temperature and the waveform 263 at the operating temperature is given as T3, and another difference in fall timing between the waveform 261 at the other reference temperature and the waveform 263 at the operating temperature is given as T4. A difference in value of electric current between the waveforms 262 and 263 is given as I1, and another difference in value of electric current between the waveforms 261 and 263 is given as I2.
A relation between the timings and the electric current data in this case is expressed as T1:T2=T3:T4=11:12=TEMP1:TEMP2, and therefore the timing and value of current at the operating temperature 263 is obtainable with linear interpolation. The head driving waveform thus becomes one illustrated as 263 shown in FIG. 14. The driving current waveform takes an area composed of an area obtained for the timing by linear interpolation and another area obtained for the value of current by linear interpolation using the AND logic, as shown in FIG. 14. A flow chart for this process is shown in FIG. 15. First, an environmental temperature (operating ambient temperature/ink temperature) is checked (S151), and a reference head driving data table is selected (S152). Next, linear interpolation for the value of current (S153) and linear interpolation for the timing (S154) are performed, and their results are composed (S155).
The foregoing techniques of the prior art impose certain problems as described below. In the case of a head drive unit having a printing cycle (T) of 25.6 microseconds and a time resolution of 0.1 microsecond, for instance, it requires data-storage means for 256 Bytes of data, such as a memory, a shift register, and the like in order to store and to output driving current waveform data enough for one complete printing cycle. It also requires means to store 256 Bytesxc3x9710, or 2560 Bytes of data, as the reference data of ten points needed for the temperature correction. In addition, it needs a data processing time for the two reference temperatures and the present temperature, for a total amount of 256 Bytesxc3x973, or 768 Bytes of data.
Furthermore, when the time resolution is increased by twofold to 0.05 microseconds to obtain the printing performance of high precision, the electric current data for one printing cycle amounts to 512 Bytes, the reference data for temperature correction amounts to 5120 Bytes, and the processing time becomes what is needed for 1536 Bytes of data. Hence, the reference data and the processing time increase in proportion to the resolution.
In other words, it is necessary for the conventional head drive unit to store and process a large amount of data for generation of the head driving waveform. It also has a problem that expands a scale of waveform-related generator circuit, and reduces the printing speed because both amount of the data and their processing time increase in proportion to resolution, when the resolution of waveform data for the head driving current is enhanced to achieve printing of high image resolution.
An object of the p resent invention is to overcoming the afore-said problems. A head drive unit for ink-jet recorder of this invention comprises:
(1) an address counter for counting reference clocks;
(2) time data storage means for storing a plurality of time data, each of the plurality of time data representing the time at a point when an electric current changes;
(3) electric current data storage means for storing a plurality of electric current data corresponding to the plurality of time data respectively;
(4) a plurality of comparators for comparing each of the plurality of time data with address count data of the address counter, each of the plurality of comparators outputs a matching signal when each of the plurality of time data matches with the address count data; and
(5) output means for storing and outputting, upon the matching signal is output, an electric current data corresponding to a time data compared by one of the comparators that outputs the matching signal,
wherein the head drive unit drives a head based on the electric current data output by the output means.
A method of driving a head of the ink-jet recorder of this invention comprises the steps of:
(a) comparing a time for head driving operation with each of a plurality of time data at points where electric current changes in a head driving waveform; and
(b) outputting an electric current data corresponding to a time data in match wit h the time for head driving operation, among the plurality of time data, when the time for head driving operation matches with any of the plurality of time data, thereby driving the head based on the electric current data output in the step (b).